This invention relates to a flat panel display device in which a passive display element such as a liquid crystal display element or the like is used and, more particularly, to an outer (ambient) light reflection type flat panel display device.
Since liquid crystal display devices (LCDs) are optically passive elements, the LCDs need illumination light sources for image display. One of the most important requirements for the LCDs is lower power consumption which primarily depends on such illumination light sources. In order to meet that requirement, reflection type and semi-transmission light type liquid crystal display devices (LCD) have been developed. Since the reflection type LCD uses outer (ambient) light as an illumination light source, its display screen becomes dark in some circumstances due to lack of illuminance. Thus, it does not work usefully in a dark place.
The semi-transmission light type LCD, on the other hand, is provided with an illumination light source and a semi-transmission light (half) mirror to reflect outer (ambient) light so that it can be used as a transmission type LCD in dark circumstances while as a reflection type LCD under sufficiently illuminated conditions. The semi-transmission light mirror in the latter, however, is 50% at maximum efficiency of outer light utilization. Its screen illuminance is significantly poor in comparison with that of the transmission type or even reflection type LCDs.
Improvement of such a technical difficulty has been recently attempted for a semi-transmission light type LCD in which a plurality of pin-holes for each pixel are provided in a reflection plate and micro-lenses are correspondingly provided for each pin hole. In this LCD, since outer (ambient) light beams reflected from the reflection plate except the pin-holes are utilized while light beams passing through the micro-lenses are collected as a light-transmission type LCD when a light source disposed at the rear of the LCD is operated, its optical efficiency becomes better. When, however, the outer light beams are utilized, an optical loss takes place at the pin-holes. As a result, this LCD is often used as the light-transmission type and its power consumption is not always saved. Further, it is necessary to additionally install the reflection plate in the device from its outside to avoid an otherwise complicated structure. The reflection plate of this kind causes a parallax effect that decreases display performance.
Still another reflection type LCD is also under development. This reflection type LCD includes a front illumination light source which consists of a light guide provided on the observer side and a linear light source provided on a side edge of the light guide. Conspicuous light reflection on the front surface of the LCD, however, causes unsatisfactory display dignity, e.g., poor contrast.